1. Technical Field of the Invention
The present invention relates to a method of magnetizing into a permanent magnet, and particularly to a method of magnetizing into a permanent magnet where while lowering the temperature of an object to be magnetized from a temperature of its Curie point or above to a temperature of below the Curie point, a magnetizing magnetic field continues to be applied to the object. This technique is effective in magnetizing a ring-like object into a multi-poled permanent magnet, which is used, for example, for a rotor of a stepping motor having a very small diameter but not limited thereto.
2. Related Art
In order to magnetize a ring-like rotor into a multi-poled permanent magnet that is incorporated in a radial-gap permanent magnet stepping motor or the like, a magnetizing device of a coil-energizing scheme is generally used. Such a magnetizing device has a structure where an object receiving hole in which a ring-like object to be magnetized into a permanent magnet can be removably inserted is made in, e.g., a magnetic yoke, where multiple grooves extending axially are formed in the inner side of the object receiving hole and where an insulation-coated conductor is laid through the grooves and the insulation-coated conductor in a winding shape forms a coil. A to-be-magnetized object is inserted into the object receiving hole, and by discharging the charge stored in a capacitor in an instant, a pulse current is made to flow through the coil, and the magnetic field created thereby magnetizes the object.
As well known, in recent years electronic apparatuses have become greatly smaller in size, and correspondingly, stepping motors and the like that are used therein have become increasingly small in size and diameter. When magnetizing into a multi-poled ring-like permanent magnet as a rotor, a large current in pulse form is made to flow with use of a magnetizing device of the above coil-energizing scheme, but as ring-like permanent magnets become smaller in diameter, the magnetization pitch (distance between magnetic poles) becomes narrower and thus the conductor of the above coil becomes thinner, thus limiting the allowable amount of current to flow through the conductor. Hence, the problem occurs that a sufficient magnetization characteristic is not obtained.
As a solution to this problem, a method has been proposed wherein a plurality of permanent magnets are arranged extending radially and thereby a plurality of opposite magnetic poles are arranged in the center and wherein a to-be-magnetized object is placed at the center, thereby magnetizing the object to be four or more multi-poled. Refer to Japanese Patent Application Laid-Open Publication No. 2001-268860. Certainly, by using such a magnetizing device of a permanent magnet scheme, the shortage of magnetization due to the magnetization pitches of magnetized objects being narrower can be alleviated to a certain degree.
However, recently the demand for stepping motors to be miniaturized and enhanced in performance is extremely high. For example, for the auto-focus mechanism of mobile image/video apparatuses, a small-pitch multi-pole magnetized stepping motor that can control a lens actuator highly accurately is an important electronic component to obtain highly fine images. Meanwhile, a magnetization characteristic of a saturated magnetization level is required of a ring-like permanent magnet as a rotor that has a small pitch structure with, e.g., 3 mm or less in diameter and the number of magnetized poles being ten or more. For such a structure, even with the above conventional magnetizing method of the permanent magnet scheme, the problem occurs that magnetization falls short and that variation between surface magnetic flux density peak values is large.
As a technique to alleviate the shortage of magnetization, a magnetizing method has been proposed which uses the fact that the magnetic field for saturated magnetization decreases in an atmosphere of high temperature or a liquid. Refer to Japanese Patent Application Laid-Open Publication No. H06-140248, which discloses that with, e.g., a Pr—Fe—B magnet that is a kind of rare-earth permanent magnet, because the magnetic field for magnetization is lower at 100° C. than at 25° C., by magnetizing at this higher temperature, saturated magnetization can be achieved with a stable low magnetic field.
However, when actually magnetized, with a ring-like permanent magnet having a narrow magnetization pitch such as the above very-small-diameter multi-poled magnet, although there is seen a slight improvement in the average of the peak values of surface magnetic flux density for all poles, variation between the peak values of surface magnetic flux density is still large. Hence, magnetization of high quality is extremely difficult.